(1) Field of the Invention
The present invention relates to a semiconductor device including: a transfer channel for transferring charge generated by photoelectric conversion; an insulating film which is formed on the transfer channel; and a transfer electrode for applying a voltage for charge transfer to the transfer channel via the insulating film, a manufacturing method of the semiconductor device, and a camera, and particularly to improvements of the insulating film.
(2) Description of the Related Art
A CCD (Charge Coupled Device) solid-state image sensor is a kind of a semiconductor device. A CCD solid-state image sensor includes a matrix of photoelectric conversion elements (also referred to as “photodiodes”), a plurality of vertical CCDs that are shift registers for reading out charge from photodiodes row by row and transferring them in the vertical direction, and a horizontal CCD that is a shift register for transferring, in the horizontal direction, the charge transferred from the plurality of vertical CCDs. Each of the vertical CCDs and the horizontal CCD includes a transfer channel that is a charge transfer channel, two-layer transfer electrodes for applying a voltage to the transfer channel, and an insulating film between the transfer channel and the transfer electrode. In general, four-phase voltages are applied to the transfer electrodes of the vertical CCD, and two-phase voltages are applied to the transfer electrodes of the horizontal CCD.
FIG. 1 is a cross-sectional view of a conventional CCD solid-state image sensor. FIG. 2 is a top view of a CCD solid-state image sensor. Particularly, FIG. 1 shows a cross section X-X on the top view in FIG. 2 showing the transfer electrodes of the CCD solid-state image sensor.
In FIG. 2, a transfer channel 2 and transfer electrodes 5 and 6 which are alternately formed thereon construct a vertical CCD. The transfer electrode 5 doubles as a readout electrode for reading out charge from a photodiode 9 to the transfer channel 2. When a readout voltage is applied to the transfer electrode 5, it reads out the charge stored in the photodiode 9 to the transfer channel 2. The charge read out to the transfer channel 2 is shifted in sequence by the four-phase voltages applied to the transfer electrodes.
In FIG. 1, the transfer channel 2 is formed by doping an n-type dopant into a p-type well 1, for example. Above the transfer channel 2, the transfer electrode 5 is formed on the insulating film 3. The insulating film 3 is made up of two layers of a silicon oxide film and a silicon nitride film. Above the transfer electrode 5, a light-shielding film 8 is formed on the protective film 7.
Japanese Laid-Open Patent Application Publication No. 05-343440 discloses an invention regarding an improvement of a shape of a transfer electrode in a conventional solid-state image sensor.
However, the above conventional art has a problem that makes it difficult to reduce the thickness of an insulating film without degradation in image quality.
To be more specific, it is preferable that the insulating film between a transfer channel and a transfer electrode is thinner for greater amount of charge storage in the transfer channel or for complete readout of the charge from a photodiode to the transfer channel without the slightest loss in the charge.
However, the thinner insulating film causes the following adverse effects. The thinner insulating film causes an increase in electric field strength between the transfer electrode and the semiconductor substrate, thus producing hot electrons that cause noises in images, and as a result, image quality is degraded. In addition, trapping of these hot electrons in the insulating film induces a threshold shift of both the voltages for charge transfer and readout. Once the threshold shift occurs, the charge is not completely transferred within the transfer channel even if the normal transfer voltage is applied, and the charge is not completely read out to the transfer channel even if the normal readout voltage is applied. As a result, the image quality is degraded as if the sensitivity of the entire image is degraded.
Furthermore, with a recent advance of fine processing technology, it has been much more likely to produce the above-mentioned adverse effects on the insulating film resulted from the increase in the electric field strength.